Organic electroluminescence is a phenomenon wherein, as electrons and holes are injected into an organic thin film, respectively, from a cathode and anode, excitons, which emit light having a particular wavelength, are generated within the organic thin film. Devices using this phenomenon have several advantages. They are lightweight, thin, self-luminous, operate at a low power, and have a rapid switching velocity.
Among such organic electroluminescent devices, a polymeric electroluminescent device, in which a thin film is formed by a spin-coating technique, as reported by a group of British researchers led by Professor R. H. Friend in 1990, merits particular attention due to its cost-effectiveness as compared to a low molecular electroluminescent device, in which a thin film is formed by a vapor deposition technique. An exemplary polymeric electroluminescent device is shown in FIG. 1. Polymeric electroluminescent device 100 comprises substrate 60, anode layer 10, hole transport layer 30, emitting layer 50, electron transport layer 40, cathode layer 20, and encapsulation layer 70. Hole transport layer 30, electron transport layer 40 and emitting layer 50 are formed between anode layer 10 and cathode layer 20 on substrate 60. Anode layer 10 is made of a compound metal oxide thin film, such as ITO (Indium-Tin Oxide), which is transparent in the visible light range and has a high work function, which facilitates injection of holes. Cathode layer 20 is generally made of an alloy of low work function metals, such as Cs, Li and Ca, and stable, high work function metals, such as Al, Cu and Ag.
When a forward bias voltage is applied across anode layer 10 and cathode layer 20, holes from anode layer 10 move into emitting layer 50 via hole transport layer 30 and electrons from cathode layer 20 move into emitting layer 50 via electron transport layer 40. Holes and electrons from anode layer 10 and cathode layer 20, respectively, have different mobility, however the mobility of holes and electrons becomes similar as holes and electrons pass through hole transport layer 30 and electron transport layer 40, respectively. The electrons moving into emitting layer 50 are trapped within emitting layer 50 by an energy barrier at the interface between emitting layer 50 and hole transport layer 30, and thus the efficiency of recombination of holes and electrons is enhanced. Consequently, the density of holes and electrons is well balanced in emitting layer 50 and improves the luminous efficiency of polymeric electroluminescent device 100. Particularly, the luminous stability of polymeric electroluminescent device 100 can be further enhanced by forming between anode layer 10 and hole transport layer 30 a buffer layer (not shown), which functions as a hole injection layer, so as to make the surface of anode layer 10 flatter.
As described above, holes and electrons injected from anode layer 10 and cathode layer 20, respectively, recombine with each other in emitting layer 50 so that excitons are generated within the luminescent polymer of emitting layer 50. These excitons are categorized into excitons in a singlet state and excitons in a triplet state (hereinafter, “singlet excitons” and “triplet excitons,” respectively). The generation ratio of the singlet excitons to triplet excitons is about 1:3. When the singlet excitons transition in energy level from the exited state to the ground state, light of a particular wavelength is emitted and luminescence is observed through substrate 60.
On the other hand, when the triplet excitons, which have a longer lifetime than that of the singlet excitons, undergo an energy transition from the exited state to the ground state, no luminescence is observed. Instead, the triplet excitons generate singlet oxygen by transferring the energy acquired by the energy transition to the oxygen existing within polymeric electroluminescent device 100. The singlet oxygen generate carboxyl groups, thereby cutting the luminescent polymeric chains of emitting layer 50, degrading the luminescent capabilities of polymeric electroluminescent device 100, and causing photo-oxidation of emitting layer 50. To alleviate this problem, polymeric electroluminescent device 100 is isolated from oxygen by forming encapsulation layer 70. Conventionally, this encapsulation technique has been generally used to prevent oxygen from penetrating polymeric electroluminescent device 100. However, the encapsulation technique has the disadvantage that it must be performed in a limited temperature range, which is lower than the decomposition temperature of polymer, and is thus difficult to apply to flexible display devices.
Another technique for suppressing the photo-oxidation of emitting layer 50 is to remove the energy of the triplet excitons causing the photo-oxidation. The energy of the triplet excitons can be removed by using an emitting layer formed by mixing metal particles or semiconductor particles, which are capable of absorbing the energy of the triplet excitons via surface plasmon resonance, with a luminescent polymer. However, those metal particles or semiconductor particles known up to now have sizes on the order of several hundred nm and are thus inappropriate for fabricating polymeric electroluminescent devices with an emitting layer of approximately 100 nm. Moreover, the fabrication process of metal particles or semiconductor particles has proven too complicated (see Hale et al., Appl. Phys. Lett., vol. 78, p 1502, 2001 and Lim et al., Synth. Metal, vol. 128, p 133, 2002).